Axially homogeneous thorium fuel designs for transuranic burning in reduced-moderation BWRs

Reduced-moderation Boiling Water Reactors (RBWRs) can allow sustained burning of transuranics (TRU), such that full actinide recycle can be achieved. However, the void coefficient (VC) tends to become positive with reduced moderation and high TRU loading, which can severely limit the design feasibility and performance. This motivates use of thorium (Th) as an alternative feed to uranium (U), as this tends to result in a more negative VC. Using coupled neutronic-thermal-hydraulic core calculations, Th-fuelled RBWRs are shown to be capable of achieving high discharge burn-ups, while incinerating an external supply of TRU. This gives them some flexibility to incinerate TRU with different isotope vectors. Spatial separation of Th-TRU and Th-U3 into regions of the order of a few thermal neutron diffusion lengths greatly improves neutronic performance. This can be accomplished radially or axially, but radial separation results in significantly easier fuel fabrication. A companion paper considers axially heterogeneous fuel designs, which are not found to deliver improved performance. A radially heterogeneous RBWR core design, can achieve 135 kg/GWthyr waste incineration rate, and at least 70–80 GWd/t discharge burn-up, depending on the TRU cooling time, and whether a single pass through a conventional PWR is used as an initial step before full recycle in an RBWR. Alternatively, a micro-heterogeneous assembly design with Th-Pu and Th-U3-MA pins can reduce fuel fabrication costs, although this limits the TRU incineration rate to ∼100 kg/GWthyr. ∼200 cm and ∼120 cm core heights are considered, with the former being generally preferred due to the possibility to match both the rating and pressure vessel size of an ABWR. (author)